The present invention relates to a field emission cathode which is a high brightness electron source, and a method for the preparation thereof. More particularly, the invention relates to a field emission cathode which can provide a high field emission stably even under a high vacuum pressure, and a method for the preparation thereof.
The field emission cathode is a cathode which emits electrons by a tunnel effect when a high electric field is applied thereto. As is well-known in the art, in the field emission cathode, as the intensity of the electric field to be applied is increased, the obtained current density can be heightened, and a current density of about 10.sup.5 A/cm.sup.2 can easily be obtained. This value of the current density is about 10.sup.3 times the practical upper limit of the current density obtainable by a so-called thermionic cathode, which is about 100 A/cm.sup.2. Therefore, many research works have heretofore been made to apply this field emission cathode to various electron beam instruments such as electronic microscopes, electron probe microanalyzers and electron beam fabrication instruments, and at the present the field emission cathode is used for some electron beam instruments.
The practical application of this field emission cathode involves a serious problem. Namely, no good current stability can be obtained unless the cathode is actuated under ultra high vacuum of the order of 10.sup.-10 Torr. In this point, the field emission cathode is very disadvantageous over the thermionic cathode which is stably actuated under a higher vacuum pressure of about 10.sup.-5 to 10.sup.-6 Torr, and this disadvantage results in increase of costs for production of an evacuation system, a vacuum instrument and the like and treatment costs.
It is known that the current density of the field emission cathode is improved as a high vacuum, i.e. a low pressure, but the reason why the stability is lowered at a low vacuum or high pressure has not been completely elucidated. Of course, it is presumed that the reduction of the stability may be caused by adsorption of residual gases at the cathode tip surface, ion bombardment to the cathode tip owing to ions which are ionized by electrons from neutral gases and migration of admolecules and adatoms, and such presumption is supported to some extent by experimental facts. However, a complete system has not yet been established for the mechanism of the above reduction of the current stability. Accordingly, although various research works have heretofore been made on the clean surface of tungsten (W) which is only one substance now practically utilized as the field emission cathode, the unstability of field emission has not been revealed.
When tungsten is actuated as a field emission cathode under ultra high vacuum of 5 .times. 10.sup.-9 to 5 .times. 10.sup.-10 Torr under such condition that extreme discharge of gases is not caused on the anode by radiation of currents, it is noted that some problems arise.
In the first place, drastic current damping is caused in the initial emission. It is understood that this is due to adsorption of molecules of hydrogen which is a major residual gas component left in a high vacuum instrument even after evacuation by an ion pump.
In the second place, the so-called stable region is changed greatly depending on the vacuum pressure and the electron bombardment at the anode, and a minute difference of the operation condition or the effective evacuating volume between the cathode and the anode results in a great difference of the current in the stable region or the term of the stable region. When the vacuum pressure is elevated, the term of the stable region is especially shortened.
In the third place, in general, the radiative angle .beta. of the field emission from a needle-shaped cathode of tungsten is as large as 1/2 rad, and the field emission pattern on the anode screen differs greatly depending on the direction of the crystallographical surface of the needle portion. In general, the aperture angle .alpha. of the small anode slit is changed according to the use of the electron probe after passage through the anode depending on the desired current density, probe size and probe current, but it is usually less than 15 mrad. Accordingly, the fact that the radiative angle .beta. of the field emission is as large as 1/2 rad means that a total emission current about 1000 times the probe current is required. The magnitude of the fluctuation of the probe current as a local current is much higher than that of the total emission current especially when the vacuum pressure is high. Even if the noise component (the magnitude of the local current fluctuation) is reduced within 5%, the term of the stable region is several hours at longest.
As will be apparent from the foregoing illustration, some difficulties are involved in taking out a current from tungsten by field emission stably for a long time even under the condition of ultra high vacuum. This is also true of metals other than tungsten, alloys and compounds more or less.
However, demand for using a high current density electron source under a higher vacuum pressure is great, and if this demand is satisfied, various effects and advantages will be attained. For example, when a needle-shaped cathode of tungsten is used under vacuum of 1 .times. 10.sup.-7 Torr, the proportion of the noise component is increased to about 100% (fluctuation equal to the measured current value) in a very short time and the needle-shaped cathode will be destroyed by discharge one to several minutes. As means for improving the stability under the condition of a higher vacuum pressure, there may be considered heating of a needle-shaped cathode. More specifically, according to this solution, admolecules are not allowed to stick on the surface of the cathode or the residence time is shortened. In short, the essence of this solution is to determine the sticking probability at a certain temperature, and some effects can be obtained according to this solution (although the effects are very low under 1 .times. 10.sup.-7 Torr, considerable effects can be obtained under a vacuum pressure of the order of 10.sup.-9 Torr). As one phenomenon seen in the field emission, there can be mentioned one in which a high field intensity is present at the tip of the needle-shaped cathode and hence, a high attractive force is imposed on the cathode tip. What resists this attractive force is the tensile strength of the cathode material. This strength is reduced by heating. Accordingly, if a needle-shaped cathode of tungsten is used under a higher vacuum pressure without heating, the cathode is destroyed by adsorption of gases, ion bombardment and finally vacuum arc discharge, and if heating is conducted, the tip of the cathode is deformed by the attractive force of the electric field and the vacuum arc discharge is caused by mechanical destruction. Because of these two destruction processes, no effective solution for stabilizing the field emission under a higher vacuum pressure has been provided.
As pointed out above, the cause of the current fluctuation (noise) in a field emission cathode has not been elucidated, but the number of factors considered to cause this undesired phenomenon is limited. Accordingly, investigations have been made to reduce influences of these factors.
(1) Gas Adsorption: PA0 (2) Work Function of Cathode: PA0 (3) Ion Etching Rate: PA0 (4) Strength to Discharge:
Apparently, there is a certain relation between the vacuum pressure and the noise in the field emission, though the mechanism has not been clarified. It is generally explained that the work function of the cathode surface is minutely changed by adsorption of gases and this minute change of the work function causes the current fluctuation. However, the effects by adsorption, desorption and migration on the cathode surface must be detailed. In case of a single crystal such as tungsten, the work function differs among respective crystallographical surfaces, and hence, also the sticking probability and the sticking energy differ. As regards adsorbed gases, it is known that adsorbed hydrogen molecules (H.sub.2) are effective for stabilizing the current but adsorbed carbon monoxide molecules (CO) enhance the current unstability.
In order to reduce the influence of gas adsorption, it is preferred to use a cathode in which the change of the work function by gas adsorption is very small, the adsorption is stronger and stable, or the adsorption is substantially reduced by heating without reduction of the tensile strength.
In general, a higher work function is preferred because a lower work function is more readily influenced by gas adsorption, and it is also preferred that the difference of the work function among crystallographical surfaces be small, because a smaller difference is more effective for reducing the effects by migration. It is preferred to use a substance having no crystal structure if possible.
In view of consumption or destruction of the cathode by ion bombardment, it is preferred that the ion etching rate (the ratio of the number of ions etched on a unit area for a unit time to the total number of ions) be low.
In order to enable field emission under a high vacuum pressure, first of all, it is necessary that the tip of the cathode should not readily be destroyed by discharge. In case of tungsten, the cathode tip is substantially completely destroyed by discharge under a high vacuum pressure and the tip is rounded. This means that tungsten is locally molten and evaporated by vacuum arc discharge. Accordingly, a substance having a very high melting point or a substance that does not melt at all meets this requirement.
A substance fully satisfying all of the above 4 requirements completely is not present at all. It is as if conductive diamond were sought for. Carbon materials have a work function of 4 to 4.5 eV and they have inevitably a low ion etching rate and do not melt under an atmospheric pressure on the earth. Accordingly, they are considerably satisfactory except the point (1). In connection with this point (1), in view of the value of the electron negativity of carbon materials (higher than that of tungsten and not so different from those of adsorbed gases), it is presumed that the influence by adsorbed gases is smaller in carbon materials, though the work function is substantially equal to that of tungsten.
The foregoing considerations are well in agreement with experimental data reported by T. H. English et al ("Scanning Electron Microscopy; System and Applications, 1973", pages 12-14. Conference Series No. 18, The Institute of Physics, London and Briston). Namely, it is reported that when a carbon fiber is used as a carbon material for a field emission cathode, a vacuum pressure of the order of 10.sup.-8 Torr is sufficient for obtaining a current stability comparable to the current stability of tungsten.
As will be apparent from the above experimental results, it is very difficult to obtain a single spot when a carbon fiber is used, and there is a disadvantage that in order to obtain a stable single point, a maximum emission current must be maintained at such a low level as several .mu.A. As pointed out by Braum et al (Vacuum, 25, No. 9/10, 1975, pages 425-426), the reason is construed to be that the carbon fiber is composed of finer fibrils. The carbon fiber has a structure in which fine fibrils are bundled along the fiber axis. Accordingly, even if a needle-shaped cathode is formed from the carbon fiber, a smooth cathode tip surface is not obtained and field emission takes place on each of tips of respective fibrils.
Further, in case of a carbon fiber cathode, since the tip surface is not smooth, the tensile strength is insufficient and the resistance to discharge is low. This specific structure of the carbon fiber is deemed to be due to the fact that since the carbon fiber is prepared by calcining at a high temperature and carbonizing a rayon or acrylic fiber, the carbon fiber has the regularity as seen in graphite along the fiber axis in interiors of fibrils.